Prima - and after trick also silent

The manual points out: In reinforced concrete buildings may have to look for the windows close, otherwise the radio signal is too weak.

I have attached my clock with a screw on a plasterboard wall and heard one seconds ticking.
The left, however, stop by, I clamped the manual behind the clock.
So probably the transmission of the sound is absorbed, and the instructions - you do not really need - is for future reference.